The accurate setting and control of the paper web tension in rotary offset presses during the printing process is of great significance. The accurately set web tension is necessary not only for achieving a good print quality, because not only are, e.g., color and crop marks better maintained, but it also leads to an increase in printing productivity, because fewer or no paper tears are caused. Defined web tension profiles are therefore usually preset along the individual paper paths, i.e., predetermined upper and lower limit values of the web tension shall not be overshot and undershot.
However, the modulus of elasticity of the paper may change greatly from one paper roll to the next, which may lead to an abrupt change in the web tension. This happens, e.g., at the time of the change of the rolls and has a highly adverse effect on the quality of the printed product. Furthermore, the modulus of elasticity of the paper may also change within the same paper roll, because the inner and outer layers of the paper roll have different moisture contents due to, e.g., storage. These changes in the modulus of elasticity of the paper lead to changes in the color and crop mark during the unrolling of the paper and consequently to an impairment in quality because of the resulting changes in the web tension and the stretching of the paper. Varying ink and moisture densities also cause changes in the modulus of elasticity.
Another influencing variable affecting the tension of the paper web are transient processes, e.g., ramp-like changes in the velocity of the paper web, or even the movement of a blanket cylinder between a print-on position and a print-off position. These transient processes frequently occur, e.g., in rotary offset presses driven in a shaftless manner with a so-called "flying" plate change functionality, where different production runs take place consecutively without stopping the printing presses. The web tension is strongly affected and changed each time here.
Since these changes in the web tension lead to an impairment in the print quality, increasingly better controls, which will be described below, were developed for maintaining the preset web tension profile.
1. Simple Web Tension Control
FIG. 4a shows a prior-art web tension control device. A web tension set point F.sub.SOLL is preset by a press control and a web tension controller determines a lag set point .DELTA.N.sub.SOLL from a difference between the web tension set point F.sub.SOLL and a web tension actual value F.sub.IST measured by a web tension measuring sensor.
A speed master set point N.sub.SOLL is picked off a folder arranged at the end of the printing process. However, since the folder already has a noncontinuous mode of operation due to its function, the speed master set point N.sub.SOLL determined from this cannot be used directly for the web tension control, but it must first be subjected to a low-pass filtration in order to suppress higher-frequency interfering components of the speed master set point signal N.sub.SOLL. The speed master set point N.sub.SOLL subjected to low-pass filtration is combined with the lag value .DELTA.N.sub.SOLL from the web tension controller and the speed actual value N.sub.IST of the roller driven by the drive motor, and the signal obtained is sent to the speed controller, which drives the drive motor.
However, the compulsory low-pass filtration of the speed master set point N.sub.SOLL from the folder is disadvantageous, because this low-pass filtration leads to an inertia of the entire control and the speed master set point intensely damped by the low-pass filtration influences the entire control dynamics of the web tension control, because the control parameters of the web tension controller must be coordinated with the control parameters of the downstream speed controller.
2. Lag Control
The lag control is a simple and rapid speed control.
As is shown in FIG. 4b, a value .DELTA.N, which is determined from a difference of a speed master set point N.sub.SOLL from, e.g., a bus system and a measured set point N.sub.IST of the speed, as well as a lag set point .DELTA.N.sub.SOLL, is sent to the speed controller. This speed controller drives the drive motor in the known manner.
However, it is necessary to set the lag set points .DELTA.N.sub.SOLL before start-up such that a desired web tension is reached, and .DELTA.N.sub.SOLL =n.multidot.N.sub.SOLL applies. Here, n denotes the lag.
Even though the lag control can be embodied in a very simple manner and it avoids the drawbacks of the web tension control that are due to the low-pass filtration, the lag control still has drawbacks. For example, the resulting web tension depends on the velocity of the paper web. This means that the web tension cannot be maintained at a constant value. e.g., during a velocity ramp, without secondary corrections of the speed master set points N.sub.SOLL. As was mentioned above, this leads to an impairment in the quality of the printed products. Furthermore, a great variation of the paper web tension has an extremely adverse effect, e.g., in the case of an normal stop or an emergency stop of the printing press, because the web tension may increase extremely greatly in the case of the pure speed control, which may easily lead to the paper web being torn off. Furthermore, the web tension is also subject to great variations during the print-on or print-off operation of all print positions of, e.g., an eight-up tower, which is likewise undesirable.
3. Lag Control with Droop Functionality
To overcome the drawbacks of the above-mentioned two control devices, a lag control with a so-called droop functionality was proposed. The speed master set point N.sub.SOLL for the speed controller of the drive of the draw-in mechanism is corrected as a function of the load moment of this drive here, the load moment being proportional under steady-state conditions to the web tension.
FIG. 4c shows such a lag control with droop functionality. A difference .DELTA.N, which is formed from a speed master set point N.sub.SOLL, a speed actual value N.sub.IST, and another correcting variable N.sub.M, which is determined from a measured motor load moment, as well as from a lag set point .DELTA.N.sub.SOLL, is again sent to a speed controller.
Contrary to a pure speed control, the control with droop functionality offers the advantage that interferences resulting from changes in the modulus of elasticity of the paper and print-on or print-off operations cause only minor deviations of the web tension. However, interferences resulting from a change in the modulus of elasticity of the paper cause a permanent deviation of the web tension unless the value of .DELTA.N.sub.SOLL is corrected secondarily. This causes a desired web tension value not being able to be maintained after an interference without a corresponding adjustment of the value of .DELTA.N.sub.SOLL, because the instantaneous modulus of elasticity of the paper web is usually unknown.
FIG. 5 shows a linearized diagram, in which the speed N of the draw-in mechanism at a certain press speed is plotted on the abscissa, and the velocity F of the paper web is plotted on the ordinate. The straight lines E.sub.1 and E.sub.2 are shown for two different moduli of elasticity of a paper web, and the modulus of elasticity of a paper web may vary between these two straight lines shown as examples. The qualitative characteristic of the simple web tension control device is designated by 1, the characteristic of the lag control is designated by 2, and the characteristic of the lag control with droop functionality by 3.
If, e.g., the modulus of elasticity of the paper web changes from E.sub.1 to E.sub.2, a difference .DELTA.F.sub.2 of the web tension is obtained in the case of a lag control (characteristic 2), and this difference is substantially greater than the difference in web tension that occurs in the case of a lag control with droop functionality, as is indicated by .DELTA.F.sub.3 in FIG. 5. This illustrates the advantage of this control.
However, even such a smaller variation in the web tension is still disadvantageous, e.g., in respect to the quality of the printed products obtained, due to the deviation of the color and crop marks.